Doctors from Britain and Cameroon have found that giving the standard drug for river blindness more frequently can reduce the symptoms of the parasitic disease. The finding could dramatically lower the number of cases, which number 18 million in Africa, Yemen and Latin America.

River blindness, formally known as onchoseriasis, is caused by a parasitic worm that lives for up to 14 years in humans. Each adult female grows to half a meter and produces millions of microscopic larvae that migrate through the body, causing rashes, intense itching, enlarged genitals, and impaired vision, including blindness.

Parasite expert Thomas Unnasch of the University of Alabama has said the standard regimen of annual or semi-annual doses of the drug ivermectin kills the baby worms, but do not harm the adults, which keep reproducing.

"So you have to keep treating until the adult female dies of old age, and that's like 14 years. That's a really long time to try and treat because many of the villages that are endemic for river blindness are very, very inaccessible in sub-Saharan Africa or South and Central America," he said.

But a three-year experiment in Cameroon among almost 700 river blindness patients has shown that administering ivermectin every three months instead of every six or 12 greatly reduced the number of female worms living within patients' skin. Physician Brian Duke of the River Blindness Foundation in Lancaster, England led the research.

"When we cut out the nodules after three years of treatment, we found that about 30 percent more of the adult worms were dead compared to the annual treatment at that dose," he said.

Dr. Duke and his Cameroon colleagues report in the journal Lancet that quarterly ivermectin doses could lower transmission of the river blindness parasite among people and lessen the duration of control programs. But determining that will require another study once the programs adopt the more frequent dosing.

"If it doesn't cut down on the amount of transmission, then we're not going to be a terribly lot better off. But if it did reduce transmission considerably, then it might be a very useful thing," he said.

University of Alabama parasitologist Thomas Unnasch said if more frequent ivermectin treatment does slow transmission, there is hope of eradicating river blindness in the Americas, which have only one percent of the world's cases. But he said this may not be possible in Africa.

"The intensity of the infection is much higher. The vector that transmits the parasite, the black fly, is extremely efficient. So it's a promising development. I think it will really help reduce illness in Africa, but whether it will actually be sufficient to break the cycle of transmission in Africa remains to be seen," Mr. Unnasch said.

The good news is that administering ivermectin more often would not be much more expensive than now. Mr. Unnasch said the drug company Merck donates it for free and the distribution infrastructure already exists.